The present invention relates to a servo-control system for the steerable front wheels of a vehicle, particularly the steerable nose gear of an aircraft travelling over the ground.
It is known that, within a given range of values of the speed of a vehicle, the steering wheels thereof are subjected to a set of complex movements which result in the floating of said steering wheels. This phenomenon is generally designated by the term shimmying and it adversely affects the efficiency and accuracy of steering said vehicle along a desired path.
Furthermore, it is known that the control devices of servo-control systems are subject, because of their very closed loop structure, to instability phenomena.
Consequently, when, as is the case for aircraft, the steerable front wheels of a vehicle are controlled by a servo-control system, the combination of the shimmying effects and the instability of the corresponding control device may make the path of the vehicle uncontrollable. Thus it may cause an aircraft travelling over the ground to leave the taxiing lane during landing or take off. The fact of leaving the taxiing lane may result in destruction of the aircraft and of course injuries, possibly fatal, to the passengers and to the crew members of said aircraft.
Very often, the servo-control system for the steerable nose gear of an aircraft is of the electrohydraulic type and attempts have already been made to overcome the above drawbacks by using, in such systems, a hydraulic jack with opening distributor slide valve. However, such a solution is not satisfactory for it can only remedy shimmying movements corresponding to small imbalances of the nose gear and is totally inefficient when considerable imbalances appear, for example in the case of stripping of a tire of said wheels. In addition, it considerably impairs the accuracy of steering by increasing the time constant of the servo-control, which is particularly disadvantageous when the steering of the aircraft is controlled by the automatic pilot.
The aim of the present invention is to overcome these drawbacks and to provide a servo-control system for the steerable front wheels of a vehicle which is at one and the same time accurate, stable and insensitive to shimmying.